A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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hawking radiation [duplicate]

My question is regarding Hawking Radiation and its effect on black hole. Now as we know that particle and particle pairs create at every place in space and it is true near black hole horizon. Then why ...
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Supernovae and black holes?

I think i am correct in saying that a supernova ($Type$ $II$) is caused by the collapse of the core of a giant star. This contraction of the core is stopped by the Pauli exclusion principle and the ...
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What happens when an electron jumps through the event horizon of a black hole?

If quantum teleportation is performed into a black hole (by an electron for example), what happens to that electron? Let's say a hydrogen atom is very close to a black hole and the electron jumps ...
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Does someone falling into a black hole see the end of the universe?

This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event ...
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Has the idea of holeum been debunked?

Here is the wikipedia article. The basic idea is that the primordial density of microscopic black holes was high enough that many were able to form stable bound states before decaying through Hawking ...
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Boundaries of Black Hole and Observable Universe

Are the boundaries of a black hole, the event horizon, similar to the boundary at the end of our observable universe?
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What happens when two high-speed objects going opposite directions crash into a black hole?

Imagine the following: A-> [] <-B where [] is a black hole, and A and B are two objects of same mass, traveling with the same speed right at the black ...
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From where (in space-time) does Hawking radiation originate?

According to my understanding of black hole thermodynamics, if I observe a black hole from a safe distance I should observe black body radiation emanating from it, with a temperature determined by its ...
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Time dilation factor for the circular orbit at 3/2 Schwarzschild radius

What would the time dilation factor be if a massive (as in rest mass>0) point particle orbiting a Schwarzschild black hole in the photon sphere? If I understand correctly, this is the only possible ...
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Angular momentum of a black hole

I recently read this Phys.SE post and, since I didn't know that black holes had a spin, a question came to my mind: how can I calculate the spin velocity of a black hole? Does mass or radius affects ...
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What is the relationship between the Schwarzschild radius and Black hole Singularity?

What is the relationship between the Schwarzschild radius and Black hole Singularity? Can the Planck length be the length of singularity? Or is the length of the Schwartzschild shorter than the ...
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Gravitational wave, Planck length and Schwarzschild radius

I have heard that Schwarzschild radius corresponds to the Planck length when the radius gain the Planck energy. Is the gravitational wave length (amplitude) equal to the Planck length?
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Does dark matter get drawn into black holes? [duplicate]

Would dark matter get drawn into a black hole? Also, could enough dark matter be concentrated to create a "dark black hole"?
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Was the mass of the universe the same when it first began as it is now? [duplicate]

I've heard that the big bang began in all places at once, but my physics teacher told my class that the universe was all once compressed into a single dense piece of matter. If my physics teacher is ...
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How can black holes be so dense?

It is said that if the Earth were a black hole, it would be the size of a peanut!? How is this density possible, are atoms really that sparse that they can be compressed so tightly? Is there some ...
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Does non-mass-energy generate a gravitational field?

At a very basic level I know that gravity isn't generated by mass but rather the stress-energy tensor and when I wave my hands a lot it seems like that implies that energy in $E^2 = (pc)^2 + (mc^2)^2$ ...
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Is a black hole a perfect black body?

A black body absorbs all light/radiation in its reach. According to basic laws of physics, the more energy a body absorbs the more it can emit. Therefore, a black body absorbs all energy directed at ...
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Photons emitted at the event horizon?

While looking through the questions, a came across a section about black holes. I immediately though; what would happen if an atom is orbiting a black hole and emitted a photon perpendicular to the ...
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If you shoot a light beam behind the event horizon of a black hole, what happens to the light?

I have a couple of questions about light here, and sorry of they are silly.. So since anything that goes beyond the event horizon can't go out, so what if a light beam was pointed somewhere behind ...
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Can He-4 atoms create black holes?

Suppose that in the intergalactic space far from any significant gravitational attractors there is a relatively small concentration of He-4 atoms. Due to gravitational attraction fermions in this case ...
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How bright was the AGN eruption at the center of the Milky Way ~2 Ma ago?

The black hole at the center of the Milky Way is believed to have erupted as an active galactic nucleus (AGN) approximately 2 million years ago, at which time it would have been visible to early ...
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Roche Limits and black holes

I am curious due to the course of the idle research that I am doing (my hobby), and I am curious for various reasons as to the answer of this question. Please forgive me in advance for possible ...
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How would one determine how old a black hole is?

Would different observers agree on the age? Or is this question nonsensical? e.g. what's north of the north pole? There are ways of estimating the ages of stellar bodies using various methods but is ...
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2 Event Horizons come into contact [duplicate]

Just a question out of curiosity, what would happen if the event horizons of 2 black holes of the same mass were to come into contact? Would both gravitational accelerations be canceled where the ...
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Mass of a black hole

I know that if a star collapses into a volume with radius less or equal to the Schwarzschild radius $r_s=\frac{2GM}{c^2}$ then a black hole is created and it has the same mass of the star that gave it ...
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What is the energy of a black hole?

This might be a stupid question but given Einstein's general theory of relativity $E = m c^{2} $ what is the energy of a black hole? Isn't the mass of a black hole infinite? Wouldn't that be infinity ...
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How does gravity escape a black hole?

My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
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Calculating the gravity gradient of a black hole [duplicate]

Given a black hole of, say, $10^8 \odot$ (solar masses), how can I calculate the gravity gradient ($\frac{\Delta g}{m}$?) at a given distance from it?
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Information Preservation and Weather Forecasting for Black Holes

Referencing Stephen Hawking's recent paper Information Preservation and Weather Forecasting for Black Holes and this question. I understand concept of holding the information on the apparent horizon ...
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Force of gravity in the event horizon [closed]

I recently posted a question about black holes and gravity (this) and I discovered the Schwarzschild radius: $r_s=\frac{2GM}{c^2}$. I thought with a friend of mine about the actual force (in Newtons) ...
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Bekenstein entropy black hole v.s Hawking entropy black hole

Historically, Bekenstein estimated the entropy associated with a black hole in 1973, obtaining: $$ S_B = \frac{\ln(2)k_Bc^3}{8\pi\hbar G}A. $$ He already acknowledges in his article that his estimates ...
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Is this derivation of Black Hole entropy viable?

This question is motivated by this one. Suppose $l$ is the minimum measurable unit of length. What is entropy of a spinless particle contained in this interval? We know that entropy of a two-level ...
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Time Reversal in a Black Hole

I had a lively discussion with a person about black holes recently, and was making the point about gravitational acceleration in GR being paralleled by speed in SR. One thing that I know people talk ...
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light entering a black hole's singularity

I already understand that light cannot escape a black hole after passing the event horizon, so please do not explain that to me. What I would like to know is this: a well known fact about light (a ...
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Would an Object Near a Pre-Blackhole Star Experience the Same Gravity as Post-Blackhole?

My question was inspired by this question, which got me thinking. According to Newton's Law of Gravitation, $$F = G\frac{m_1m_2}{r^2},$$ the gravity of an object is inversely proportional to the ...
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Does the mass of a star change as it collapses into a black hole?

I know (I think!) that when a really big star collapses on itself it creates a black hole. My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or ...
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Why can't light escape from a classical black hole?

Photons do not have (rest) mass (that's why they can move at speed of "light"). So, my question is how the gravity of classical$^1$ black hole can stop light from escaping? -- $^1$ We ignore ...
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Are gravitons bound by the event horizon?

I understand that photons, even when traveling at the speed of light, cannot escape the event horizon of a black hole. Are gravitons and other virtual particles traveling at the speed of light also ...
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Could dark energy make a large black hole less black?

Theoretically, if a black hole grew to a huge scale such that the effect of dark energy was large, could the black hole become 'normal' space again (i.e. no horizon or singularity)? What I'm trying ...
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Photons and Black holes

How many photons in one Planck volume would it take to form a tiny black hole? A photon doesn't have mass but it does have energy, $1.0101 \times 10^{-37}$ Joule for red $650$ nm wavelength light if ...
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How does negative energy from Hawking Radiation cause a Black Hole to shrink? [duplicate]

Hello this is expanding upon a question that was previously asked on stack exchange that I linked below. From what I understand of Black Holes is that overtime Hawking Radiation results in the Black ...
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Whether the holographic universe of string theory prove the hypothesis of the holographic universe created by David Bohm or not?

Bohm, David (1980), Wholeness and the Implicate Order, London: Routledge, ISBN 0-7100-0971-2 As we can see from the book above, David created the hypothesis of the holopraphic universe in 1980. And ...
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Why does Matter occupies space? Matter and space are two things… If Space does not has mass, why should matter have space-ful nature?

This thought has completely changed my perspective towards matter. If the matter in a star can collapse to a point to form a Black hole, surely the true nature of matter should be able explain this ...
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Field equations in extended EH-GHY action. Is Schwarzschild a solution?

When taking the EH action, $$S_{EH} = \frac{1}{16\pi G}\int_M d^4x \sqrt{-g}R$$ and making a small variation in the metric while ignoring boundary terms, we obtain $$\delta S_{EH} = \frac{1}{16\pi ...
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SPEED of LIGHT as photons reach black holes [duplicate]

As photons reach a black hole and begin fall in - is their speed increased as they go down (like a vacuum cleaner does to dust).
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Speed of light originating from a star with gravitational pull close to black-hole strength?

Imagine you have a star which is on the brink of turning into a black hole. Lets say it is infinitely close to become a black hole, but not there yet. Since there is no event horizon, but a great ...
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At which point of the universe $R_{\mu \nu}=0$ if there is a source of gravitation (point mass)

Schwarschild found his solution to Einstein's field equations for vacuum ($T_{\mu \nu}=0$) by placing a point-mass in the center of origin. Since the Ricci tensor $R_{\mu \nu}$ and the Einstein ...
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Would gravity on the surface of a planet which is being consumed by a black hole change?

Assuming that the black hole starts grow in the exact center of the planet and that the general structure of the planet does not degrade as it is eaten from the inside, would the gravity on the ...
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Kinetic energy of a photon and Schwarzschild radius

I have read here, that $\frac{1}{2}mv^2$ must not be applied on a photon ever. If i want to calculate escape velocity $v_e$ i need to use $\frac{1}{2}mv^2$ because we say that kinetic energy ...
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How exactly does time slow down near a black hole?

How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...